A method is known for producing a magnetic recording medium comprising spreading a magnetic coating solution on a non-magnetic support base, drying the coating adequately and passing the base between calender rolls. The magnetic coating solution contains a binder, such as a cellulose derivative, vinyl chloride/vinyl acetate copolymer, polyurethane resin, acrylic resin, copolymer of acrylic acid or methacrylic acid and its ester, vinylidene chloride copolymer, synthetic rubber or polyester, and these binders are used alone or in combination. A technique is known for hardening the coating film by using both nitrocellulose and polyol or polyisocyanate as a binder. The coating solution containing these two compounds as a binder is spread onto a non-magnetic support base, dried and a roll of the resulting film is left before it is calendered to provide a smooth surface. This conventional method uses reaction resins (resins that harden by reacting with each other) as a binder to form a magnetic layer having increased mechanical strength. However, if the support base coated with magnetic coating solution containing such binder is left for a while, the curing reaction goes on to make the subsequent smoothing treatment by calendering difficult or inadequate. To solve this problem, the pressure or temperature of the calender rolls must be increased, but then, the life of the roll is reduced significantly by the increased roll pressure or temperature.
Nitrocellulose has high glass transition point and softening point and has long been used as a binder for magnetic tape. Magnetic particles are easily dispersed in nitrocellulose and hence, the coating solution containing nitrocellulose provides a smooth coating. But one of the recent requirements with magnetic tape is higher density of magnetic particles. To meet this requirement, a magnetic layer that has an even smoother surface and which is less susceptible to temperature and moisture must be developed. One candidate is a magnetic recording medium of the type wherein a binder resin such as the nitrocellulose in the magnetic layer is hardened. However, to produce such medium, the temperature and pressure of the calender rolls must be very high. As mentioned above this reduces the life or durability of the rolls. In the conventional method of producing magnetic tape, a web of the non-magnetic support base coated with magnetic coating solution is dried thoroughly, wound into a roll, and left for a while before it is passed between calender rolls. If the magnetic coating solution contains nitrocellulose, polyol and polyisocyanate as a binder, the resulting magnetic coating solution hardens so quickly that the web must be calendered at a very high temperature and pressure.
These conventional methods for producing magnetic recording media and tapes, the binder used therefor and the hardening techniques as discussed above are described in U.S. Pat. Nos. 4,172,176, 3,597,273, 3,216,846, 3,150,995, 4,049,871, 4,152,484, 4,115,290, 3,149,995, 3,926,826, 4,020,227, etc.